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Introduction and General Information

Prevention and control of arboviral diseases is accomplished most effectively through a comprehensive integrated mosquito management programme; effective mosquito control begins with a surveillance program that targets pest and vector species, identifies and maps their immature habitats by season, and documents the need for control. (D67)
  • Vector control is most effective when directed at the species responsible for virus transmission. For optimum impact both enzootic/epizootic and epidemic vectors should be targeted. (P32.1.w3)

Insecticides have been used widely in the reduction of populations of vector arthropod species for control of vector borne diseases such as malaria, yellow fever, epidemic typhus and plague. Whenever the use of insecticides is considered it is important to be aware of the potential hazards of their use, both in terms of human health risks and environmental impacts. (D71

  • The use of chemicals (pesticides) to control adult or immature (larval and pupal) mosquito populations may be required when other forms of control such as source reduction are not feasible or when they have failed due to unavoidable/unanticipated problems. 
  • Decisions to use larvicides or adulticides should be based on surveillance data. (D73, W175.Nov01.wnv1) 
  • The use of adulticides may be required when surveillance shows a threat to human health posed by the presence of infected adult mosquitoes. (D67)

(D67, D73, W175.Nov01.wnv1) 

Organisations responsible for mosquito control operations should prepare, in advance of any use of chemical agents, "spray maps" for both larvicide and adulticide use, delineating both possible target areas and areas which should be avoided in possible applications of insecticides. Detailed maps with individual blocks numbered may be required for control programs in urban areas. (D70, D72)

  • It is essential to remember at all times that the use of insecticides is accompanied by risks to other organisms and to the function of ecosystems. See section below: Effects on Non-target Organisms. Precautions must be taken when using insecticides to maximise effectiveness while minimising undesirable side effects such as killing of useful non-target species. (D72, D71)
  • Before any insecticides are used it is important to ensure that they are registered for use in a given location, that any laws or regulations regarding pesticides are complied with, and that required permits for their use have been obtained. (D72)
  • "All applicators and operators in most states are required to be licensed or certified by the appropriate state agencies." (D67)
  • Proper use of chemical control within an integrated mosquito management plan includes adequate surveillance of the effect of the insecticide application. (D67)

Personnel involved in mixing, loading and applying concentrated insecticides use personal protective equipment to avoid exposure to the chemicals, as well as closed systems for pumping insecticides from storage to spray equipment. (J84.7.w35)

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Chemicals used in Mosquito Control

Detailed information is also available in the following chemical sections:

Currently available adulticides include organophosphates such as malathion and naled, natural pyrethrins, and synthetic pyrethroid-based  insecticide (permethrin, resmethrin and sumithrin). Whenever possible it is advisable for responsible authorities to decide in advance which insecticides will be considered for use, taking into account technical information regarding choices of pesticides which may be used in various situations. (D67, D72)

Available larvicides include temephos (Abate), which is an organophosphate, methoprene (Altosid), which is an insect growth regulator, oils such as the petroleum based Golden Bear and the mineral based Bonide, and (for use in some limited habitats) diflubenzuron (Dimilin), which is a chitin synthesis inhibitor. (D67)

  • Biochemical larvicides (insect growth regulators - IGRs) such as methoprene can be used in a variety of habitats, and are especially effective in salt marshes, although sustained release methoprene products are not for use in known fish habitats. (D72)
  • Solid, time-release pellets and briquettes of insect growth regulators can provide sustained control over larvae in small areas, reducing labour costs, although they are in themselves expensive. (D72)
  • Monomolecular surface film products are effective against all aquatic stages of mosquitoes and can be used in polluted or artificial habitats. However they are not suitable for use where non-target insects are important resources, which eliminates the possibility of their use in most natural areas. (D72)
  • Use of the microbial larvicides Bacillus thuringiensis var. israelensis and Bacillus sphaericus is discussed under Biological Control for West Nile Virus.

Brief information on pesticides used for the control of mosquitoes in the USA:

  • Temephos (Abate) is available as granules or as an emulsifiable concentrate. Applied against larvae, its major advantage is that it is usually the lowest cost chemical. Its limitations include non target effects and some resistance.(J84.7.w35)
  • Methoprene (Altosid) is available as granules, briquettes, pellets, and liquid concentrate. Applied against larvae, its main advantages are that residual briquettes are available and its safety against non-target organisms. Its main limitation is that longer is required before its efficacy may be determined than with other larvicides therefore a longer period of surveillance is required.(J84.7.w35)
  • Diflubenzuron is a chitin-synthesis inhibitor which acts on contact to disrupt chitin synthesis and therefore the larva's moulting process. Applied against larvae it is available as a wettable powder. Its main limitation is that it also affects other arthropods therefore cannot be used in many habitats such as marshes in which crustaceans are a valuable component of the fauna, and because it reduces the number of aquatic insects it may have secondary effects on populations of insectivorous birds and fish. (P32.1.w8, D73)
  • Oils (BVA, Golden Bear) are available as an oil and are applied against larvae and pupae. Their main advantage is that they are active against pupae, while their main limitations are the production of an oil field and lack of effect against subsurface larvae. (J84.7.w35)
    • Oils act in part by forming a barrier which inhibits respiration of the target insects. They can cause some toxicity to plants but are generally safe to use for both the applicator and the environment. (P32.1.w8)
  • Monomolecular film (Agnique) is available as a liquid. Applied against larvae and pupae, its main advantage is that it is active against pupae while the main limitation is a lack of activity against subsurface larvae.(J84.7.w35)
    • This film apparently makes it difficult for larvae and pupae to attach to the surface, and also blocks breathing tubes, drowning the larvae/pupae. It is also claimed to drown resting male mosquitoes and egg-laying females coming into contact with the film. (P32.1.w8)
  • Malathion (Fyfanon, Atrapa, Prentox) is available in Ultra-Low-Volume and thermal fog formulations. Applied against adults, its main advantage is its tolerance (the maximum limit of pesticide residue considered safe) while its main limitations are that it is an organophosphate and some resistance. An application rate of 3 fluid ounces per acre (219.8 ml per hectare) is appropriate for control of adult mosquitoes.(J84.7.w35)
  • Naled (Dibrom, Trumpet) is available in Ultra Low Volume, emulsifiable concentrate and thermal fog formulations. Applied against adults, its main advantage is its tolerance while its main limitations are that it is an organophosphate and that it is corrosive.(J84.7.w35)
  • Fenthion (Batex) is available in Ultra Low Volume, emulsifiable concentrate and thermal fog formulations. Applied against adults, it has no specified advantages, while its main limitations are that it is only used in Florida, it is a Restricted Use Product, and its tolerances.(J84.7.w35)
  • Permethrin (Permanone, Aqua Resilin, Biomist, Mosquito-Beater) is available in Ultra Low Volume, thermal fog and clothing treatment formulation. Applied against adults and as a clothing treatment against ticks and mosquitoes, its main advantage is a low vertebrate toxicity and it has no specified limitations.(J84.7.w35)
  • Resmethrin (Scourge) is available in Ultra Low Volume and thermal fog formulations. Applied against adults, its main advantage is low vertebrate toxicity while its main limitations are that it is a Restricted Use Product and has no tolerance given for residue on crops.(J84.7.w35)
    • Due to its persistence it is considered by regulatory agencies to be likely to have undesirable effects on non-target aquatic organisms. Therefore its use near water is restricted to operators who are specially licensed and trained. (P32.1.w8)
  • Sumithrin (Anvil) is available in Ultra Low Volume and thermal fog formulations. Applied against adults, its main advantage is low vertebrate toxicity while its main limitation is that it has no tolerance.(J84.7.w35)
  • Pyrethrins (Pyrenone, Pyronyl) are available in Ultra Low Volume and emulsifiable concentrate formulations. Applied against adults and larvae, their main advantages are that they are natural pyrethrums, and their tolerances, while their main limitation is that they may be costly.(J84.7.w35)

    (J84.7.w35, P32.1.w8)

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Use of Larvicides

Controlling mosquitoes at the larval stage is the most effective and economical method to reduce contact with mosquitoes and thereby prevent and control WNV infection. (J115.13.w2)

"An effective larviciding program is an important part of an integrated mosquito control operation." (D67)

  • Larvicides are used to control mosquitoes in the aquatic stage within their breeding habitats, before they become dispersed, biting adults, and to keep mosquito populations at sufficiently low levels that the risk of transmission of arboviruses is minimal. (D67, D73, J115.13.w2)
  • In general, larviciding is more effective and target-specific than adulticiding and has less effect than adulticides do on non-target species and the environment. They are generally applied over relatively small areas (in the breeding habitats where larvae are concentrated) compared to adulticides (which must act against a dispersed population). (D67, D72, P32.1.w8, P32.1.w27, W175.Nov01.wnv1)
  • Larvicides are extremely important in areas which cannot be drained or filled readily and where it is not appropriate to introduce larvivorous fish. (D70, P32.1.w8)
  • Larvicide use can greatly reduce or even eliminate the need for adulticides later in the mosquito season. (P32.1.w8, D73)
  • For WN virus, dead bird density, plotted geographically, may be used to assist in directing use of larval mosquito control. (P39.3.w12)

Appropriate Use of Larvicides and Mechanics of Larvicide Use:

Data from mosquito surveillance is vitally important in determining when and where larvicides are to be applied. See: Mosquito Surveillance for West Nile Virus

  • It is important that larvicides are applied accurately to cover the whole breeding habitat of the target mosquitoes; even a relatively small area missed during the application of adulticides may lead to a large emergence of adult mosquitoes, necessitating broad-scale adulticide use for control.
  • Target application sited for larvicides may include irrigation ditches, flood waters, standing ponds, woodland ponds, flooded pastures, marshes, roadside ditches and storm water retention ponds. (D73)
  • Larvicides may be applied from the ground or, if the area to be treated is large or inaccessible, aerial application of larvicides may be used.
    • For ground application a variety of applicators may be used depending on the formulation of larvicide (e.g. liquid or granule) and the size of the area to be covered. For liquids, applicators vary from air-pumped "hand sprayers" through backpack sprayers to truck-mounted sprayers. For granules, similarly, small areas may be covered using shoulder-carried crank-operated dispensers, with powered backpack blowers for larger areas and truck-mounted blowers for treatment of areas such as wide roadside ditches. (D73)
    • Liquid formulations of larvicides are rarely made from the air because mosquito breeding sites are generally heavily vegetated or in wooded areas, so most droplets would be likely to catch on vegetation and never reach the target water pool. (D73)
  • Larvicides should be specific for mosquitoes, with minimal impacts on non-target organisms.
  • In many habitats it is essential that the formulation of larvicide chosen is capable of penetrating dense canopies of vegetation.
  • Where aerial application is used, accurate guidance, using simple methods such as flags on poles or more sophisticated methods such as GPS, produces more efficient and effective dispersal of chemical. (W175.Nov01.WNV1)

(D67, D73, W175.Nov01.wnv1)

Choice of Larvicides:

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Use of Adulticides

Use of adulticides to control adult mosquito populations is generally reserved as a last resort. (J115.13.w2)
  • "Mosquito adulticides should be considered the least desirable method of control and only used when current isolations of virus and/or evidence of disease has been established." (D72)

Appropriate Use of Adulticides:

Although other components of an integrated mosquito management programme should generally avoid the production of large mosquito populations, use of adulticides may become necessary due to a wide variety of events including heavy precipitation, flooding, high tides, environmental constraints, inaccessible larval habitats, missed breeding sites, human disease outbreaks, budget shortfalls, absent employees, or equipment failures. (J84.7.w35) It is recognised that although adulticiding is usually the least efficient mosquito control technique, nevertheless this form of control, based on surveillance data, is an extremely important part of any integrated mosquito management program. (D67)

  • Emergency vector control (spraying), if is to have an effect in reducing cases, must be applied at the start of the epidemic, not after the epidemic is well underway. (P48.4.w14)
  • It is important to remember that delaying adulticide applications in areas with surveillance indicators indicating sustained or intensified virus transmission in an area and infection with WN virus of potential accessory vector mosquito species (for example, those demonstrating mammal host ranges), until human cases occur, negates the value and purpose of the surveillance system. (D67)
  • Use of adulticides can be warranted when other means of control have been ineffective and there is a substantial risk of disease transmission. (P32.1.w8)
    • A high level of protection may be provided in the face of an epidemic, or when an epidemic threatens, by using insecticides to eliminate vectors. It is important to recognise that outdoor space treatments (such as the use of ultra low volume (ULV) application of insecticides at high concentration, spread by aerosol over a wide area) may be extremely important for rapid effect to control an epidemic. It is also important to recognise that their effect is short lived and repeated application is required for continued control. (D71)
  • Area-wide uses of adulticides can often be avoided; application may be targeted to areas where WNV-infected mosquitoes have been detected by surveillance efforts and to nearby border areas. (P32.1.w8)
  • For WN virus, dead bird density, plotted geographically, may be used to assist in directing use of adult mosquito control. (P39.3.w12)

Mechanics of Adulticide Use:

The choice of adulticide and the time of application should be based on the distribution and behaviour of the target mosquito species. Most Culex spp. are nocturnal, which may compromise aerial application in urban areas. (D67)

  • The mosquito species to be targeted must be identified. (P48.4.w14)
  • Decisions on exactly where to spray should be made on the basis of local data; in Florida sentinel chicken coops with attached mosquito traps have been used to provide information on mosquito infection rate and species involved. (P48.4.w14)
  • Meteorological conditions must be considered for timing of spraying. In general late afternoon, early evening, night or early morning, when the air is cool and wind velocity low (less than about six miles an hour) is preferred. (D70)
  • For Culex spp. focal area spraying at night using truck-mounted equipment may be more effective than application during the afternoon or at dusk. (P32.1.w23)
  • It is important to recognise that multiple applications of adulticides may be required for adequate control of vectors such as Culex pipiens. (D67)

Adulticides must drift through the habitat in which mosquitoes are flying in order to provide optimum control. They are typically applied as an Ultra-Low-Volume (ULV) spray where small amounts of insecticide are dispersed either by truck-mounted equipment or from fixed-wing or rotary aircraft. Ground or aerial applied thermal fog applications of adulticides are used in some areas, but to a much lesser degree. (D67

  • Thermal fogging reduces visibility but may penetrate vegetation better than a ULV spray application and the fog is easily tracked downwind to target areas. (J84.7.w35)

Other uses of adulticides include residual spraying of the walls of catch basins and, in some circumstances, vegetation around dwellings or parks, playgrounds etc. (D70)

Considerations in Chemical Control of Adult Mosquitoes:

  • It is recognised that there is serious public concern regarding the use of insecticides. Public education programs, to distribute information about the nature of mosquito-borne disease and about the risks and benefits of adulticide use, are vital; public acceptance is critical for the application of emergency adult mosquito control. (D67)
    • Public concern regarding the possibility of becoming ill due to the use of pesticides may be at a similar level to concern regarding becoming ill due to infection with WNV infection. (P39.3.w12)
  • Application of insecticides over water bodies should be avoided where there is a low dissolved oxygen content (less than 2 ppm) as fish deaths are likely to occur under these circumstances. (P32.1.w23)

Two examples of considerations in decision making regarding chemical control of adult mosquitoes are given below.

1) The following information regarding decision making in mosquito adulticiding operations is quoted directly from the CDC Epidemic/Epizootic West Nile Virus in the United States: Guidelines for Surveillance, Prevention and Control 3rd Revision (D147): [Text copied directly]

Once arbovirus activity is detected in a jurisdiction and a decision is made to implement or intensify mosquito control by using adulticides, the size of the area to be treated must be determined. In the broadest context, the underlying program objective (i.e., interruption of the enzootic transmission cycle vs. prevention of transmission to humans and domestic animals) should determine the amount of adulticide coverage that is required. For most jurisdictions the objective is the prevention of transmission to humans and domestic animals. There is no simple formula for determining how large an area to treat around a positive surveillance indicator or a suspected or confirmed human case of WNV. Nor is there adequate information to guide decisions about the degree of vector population suppression that must be attained, or for how long this suppression must be maintained to reduce human disease risk. At a minimum, the following factors must be considered when deciding the scope of the adulticiding effort:

1. The general ecology of the area, e.g., key habitat types and the presence of natural barriers such as large rivers;

2. The population density, distribution, flight range, and age structure (proportion of parous females) of the target mosquito species;

3. The flight range of the avian amplifying host(s);

4. The length of time since birds started dying or became infected in the affected area (typically, there may be a lag of several weeks between recovery of dead birds and confirmation of WNV infection) or since virus-positive mosquito pools were collected;

5. The human population characteristics spatial distribution and density relative to the positive locality (e.g., urban vs. rural), age demographics;

6. Evidence of persistent WNV activity detected by the surveillance program; and

7. Season of the year and how long WNV activity can be expected to persist until the epizootic/epidemic vector(s) enter diapause.

Several of these factors will be unknown or poorly understood. Technical assistance from a mosquito control professional, particularly one experienced in mosquito control in the region, is crucial in this process. Practical experience in conducting mosquito control is required to refine control recommendations. For example, the size of an area selected for control applications may be reduced in response to structures like open areas, bodies of water, major highways, or other barriers that may restrict the distribution of targeted species. Alternatively, adulticide coverage may be expanded to cover large urban or suburban residential neighborhoods with dense human populations.

Hypothetically, in some settings where focal early season enzootic WNV activity has been detected, early season adulticiding may be useful in interrupting virus transmission and lead to lower transmission rates later in the season. However, effective larval control of the principal enzootic mosquito vector is probably a more costeffective way to interrupt early-season virus amplification.

2) The following Considerations for Adult Mosquito Control are quoted directly from New York State West Nile Virus Response Plan - Guidance Document (D72) Appendix B: [Text copied directly]

West Nile Virus: Considerations for Adult Mosquito Control

The main objective of mosquito control is to decrease the risk of a human outbreak of West Nile virus (WNV) infections. This should be primarily accomplished by:

Continuing to stress reduction in mosquito habitats;

Larviciding where feasible and practical;

Using personal mosquito protection measures, especially for the elderly and immunocompromised.

Adulticiding is supplementary to these measures and is a local decision that should be based on the considerations listed (in no particular order) below.

Triggers for Spraying

Adulticing should be considered only when there is evidence of WNV epizootic activity at a level suggesting high risk of human infection (for example, high dead bird densities, high mosquito infection rates, multiple positive mosquito species including bridge vectors, horse or mammal cases indicating escalating epizootic transmission, or a human case with evidence of epizootic activity) and abundant adult vectors. In general, the finding of a WNV positive bird or mosquito pool does not by itself constitute evidence of an imminent threat to human health and warrant mosquito adulticiding.

When to Spray

The goal of spraying is to reduce the risk of human disease by decreasing the number of vector adult mosquitoes as much as possible. The pesticide is effective only when it makes contact with a mosquito. This is most likely to happen when mosquitoes are actively flying. To this end, it is imperative that spraying take place when Culex [Culex spp.] mosquitoes—the primary target mosquitoes—are most active. This means spraying between dusk and dawn. It is also important to note when spraying will NOT be effective. Spraying during inclement weather--rain, fog, high winds--will simply knock the pesticide mist to the ground, rendering it inactive. In addition, spraying at temperatures below 55-60 degrees is less effective. At these temperatures, mosquitoes are slower and are usually resting on the ground in the shrubbery and leaf litter. The pesticide mist usually can’t reach them there.

Where to Spray

Another consideration is the terrain in the proposed spraying area. If there is substantial vegetation bordering the roads, ground spraying with trucks may not provide adequate coverage. Dense vegetation associated with roadside trees, shrubs, or hedges can interfere with truck-mounted insecticide applications. In situations such as these, counties that choose to spray may wish to consider application of pesticides using backpack sprayers, or altering their route so that trucks can more efficiently apply the spray. Aerial application may be considered when all other methods of application are inadequate and/or inefficient. Aerial spraying should be limited to the immediate area where the vector population has been documented to exist through vector surveillance and to adjacent areas considered at risk for imminent disease transmission. You can contact your local DEC [Department of Environmental Conservation] office for guidance in situations involving spraying near water or with applications that might be hindered by vegetation.

Human Population Density

The population density in an area where there is evidence of intense epizootic activity should also be taken into consideration. If the area is rural and there are few people, the cost and potential risks of spraying may not justify its use. If the area is heavily populated, you have stronger indications for considering adult mosquito control, since the goal of spraying is to minimize the risk of a human outbreak of West Nile virus infection.

Mosquito Population

Information from mosquito surveillance can be helpful in determining when to conduct mosquito control, and in monitoring the effectiveness of control activities. While all mosquitoes do not need to be tested specifically for the presence of West Nile Virus, those that are tested can provide valuable information regarding spraying decisions. Be aware that surveillance efforts to detect virus in birds are much easier to conduct than similar efforts to detect virus in mosquitoes. What may be more important than testing mosquitoes for West Nile Virus is knowing the NUMBERS and SPECIES of the vector population in the locality. The best way to do this is by mosquito trapping. Systematic mosquito trapping, however, requires specially trained staff and is time intensive. For localities without this capacity, there are other potential sources of information on mosquito activity. Staff can visually inspect the area where a positive bird was found, or around human population centers for habitats likely conducive to mosquito breeding. Staff can also personally observe mosquito activity.

Lag Time

It is important to look at the dates that the positive surveillance specimens (mosquitoes, birds, and/or mammals) were collected. In most cases the positive specimens will have been collected about two weeks before. In the time between the date the specimen was collected and the date when the test results are complete, circumstances may have occurred which would alter a decision to spray. For example, a county may have sprayed since the collection date, a weather event may have adversely affected mosquitoes, or mosquito habitat may have been modified resulting in a reduced need to spray.

West Nile Surveillance Results Over Time

WNV surveillance information may be monitored by county or even smaller jurisdictions, such as towns, over time, to determine what is happening with the outbreak. For example, if there has been a consistently good system for recording dead crow sightings, and the number of dead crow sightings drops for several weeks in a row after spraying, that may indicate that the previous spraying has killed off a large enough number of mosquitoes that transmission to crows is not continuing. Such analyses should not be graphed by day (because of day-to-day instability in reporting), but analysis by week should be helpful.

Local Perspectives on Spraying

Different communities have varying perspectives on the benefits of mosquito control. These should be taken into account in the decision whether or not to spray. This can be difficult, as people can have strong opinions on both sides of the issue. The US Environmental Protection Agency has approved these chemicals for use and they must go through rigorous testing in order to get that approval. 

For further information about toxicity of the common pesticides used for adult mosquito control, please consult your local DEC regional office.

Whether or not you choose to spray, it is recommended that you contact your regional DEC office and start to get some of the paperwork together regarding pesticide application permits, etc. You may also want to call some of your colleagues at other counties for advice—what did they do in the same situation, how did their bidding process for pesticide applicators go, etc. 

The decision regarding spraying is basically a risk assessment: whether or not you, as a county, believe the risk of contracting West Nile Virus is greater than the risk from applying pesticides for mosquito control. It is also a cost assessment where you must take into account medical costs, life years lost (for WNV fatalities), costs of spraying campaigns, etc. As with any decision about access to health prevention and care, many factors must be considered. Hopefully, the above list of factors will assist in your local decision making process.

If you have any questions, and wish to discuss your situation further, please feel free to call the Arthropod-Borne Disease Program at 518-474-4568.

Barrier and Local Use of Adulticides:

  • Within houses, mosquitoes may be killed using appropriate adulticides such as synergised pyrethrum or synthetic pyrethroids sprayed using a household aerosol space spray (used according to label directions). (W175.Nov01.WNV2)
  • In some areas of the USA barrier treatments, which are typically applied as high volume liquids with hand-held spray equipment using compounds with residual characteristics, are commonly applied. The use of barrier treatments may be particularly attractive to individual homeowners living near mosquito producing habitats where residual chemicals applied along a property border can provide some control benefits. (D67)
  • Local reductions of mosquitoes around houses may involve the use of adulticides, particularly products containing allethrin, Malathion or carbaryl, sprayed with a compressed air sprayer onto vegetation, walls and other potential mosquito resting areas (used according to label directions). (W175.Nov01.WNV2)
    • Residual insecticides may repel or kill mosquitoes. Microencapsulated formulations are the most durable. Pyrethroids such as lambda cyhalothrin, detamethrin and permethrin are the most effective repellent insecticides. (N11.30.w1)

Control of Adult Mosquitoes Overwintering in Hibernacula:

  • There is a need to investigate effective methods to control overwintering mosquitoes (Culex spp. and Anopheles spp. mosquitoes overwinter as adults) in hibernacula, and, if appropriate, apply these methods judiciously in established adult mosquito resting areas. (D72)

Evaluation of Adult Mosquito Control:

  • The effectiveness of adulticiding programmes should be monitored.
  • Evaluation of adult mosquito control should include as a minimum, surveillance to measure pre- and post-spray mosquito densities inside and outside the control area using CO2-baited traps and gravid traps and vector mosquito infection rates pre- and post- spray inside and outside the control area, as well as weather conditions during application (temperature, wind speed, wind direction). (D67, D147)
  • Small areas can be selected for assessment of effectiveness, using landing counts, before and after treatment. Typically treatment is expected to reduce the landing count by greater than 85%. (D73)
  • Additional desirable data includes the population age structure of key mosquito species (e.g. Culex pipiens), data on the performance of individual pieces of application equipment (such as droplet size of ULV sprays, and flow rate) and monitoring of the spray track by GPS if equipment is available on aircraft.(D67, D147)

(D67, D73, D147)

Evaluation of effect of Adulticides on non-target Organisms:

  • The local effects of adulticide treatments on non-target organisms such as fish and lobsters should be evaluated. (P32.1.w23) See section below: Effects on Non-target Organisms
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Management of Resistance

Consideration of the potential development in vector species of resistance to chemical control agents and management to prevent or delay the development of resistance should be an integral part of any plan involving the use of chemical control of vectors.
  • Resistance management techniques aim to minimise the risk of mosquitoes becoming resistant to existing chemical used for mosquito control. (W175.Nov01.wnv2)

The following description of resistance management is taken directly from CDC Epidemic/Epizootic West Nile Virus in the United States: Revised Guidelines for Surveillance, Prevention and Control (D67): [Text copied directly]

In order to delay or prevent the development of insecticide resistance in vector populations, integrated vector management programs should include a resistance management component (modified from Florida Coordinating Council on Mosquito Control, 1998). Ideally, this includes annual monitoring of the status of resistance in the target populations to:

1. Provide baseline data for program planning and pesticide selection before the start of control operations.

2. Detect resistance at an early stage so that timely management can be implemented (even detection of resistance at a late stage can be important in elucidating the causes of failure of disease control; however, in such cases, management options other than replacement of the pesticide may not be possible).

3. Continuously monitor the effect of control strategies on resistance. In addition to monitoring resistance in the vector population, the integrated program should include options for managing resistance that are appropriate for the local conditions. The techniques regularly used are:

1. Management by Moderation - preventing onset of resistance by:

a. Using dosages no lower than the lowest label rate to avoid genetic selection.

b. Using less frequent applications.

c. Using chemicals of short environmental persistence.

d. Avoiding slow-release formulations.

e. Avoiding the use of the same class of insecticide to control both adults and immature stages.

f. Applying locally -- Currently, most districts treat only hot spots. Area-wide treatments are used only during public health alerts or outbreaks.

g. Leaving certain generations, population segments or areas untreated.

h. Establishing high pest mosquito densities or action thresholds prior to insecticide application.

i. Alternation of biorational larvicides and IGRs annually or at longer intervals.

2. Management by Continued Suppression - a strategy used in areas of high-value (e.g., heavy tourist areas in the case of mosquito control) or where insect vectors of disease must be kept at very low densities. 

This does not mean saturation of the environment by pesticides, but rather the saturation of the defense mechanisms of the insect by insecticide dosages that can overcome resistance. This is achieved by the application of dosages within label rates but sufficiently high to be lethal to susceptible as well as to heterozygous-resistant individuals. If the heterozygous individuals are killed, no resistance will occur because homozygous- resistant individuals do not exist or they are at such a small frequency that quick population build-up is unlikely. This method should not be used if any significant portion of the population in question is resistant. Another approach more commonly used is the addition of synergists that inhibit existing detoxification enzymes and thus eliminate the competitive advantage of these individuals. Commonly, the synergist of choice in mosquito control is piperonyl butoxide (PBO).

3. Management by Multiple Attack - achieving control through the action of several different and independent pressures such that selection for any one of them would be below that required for the development of resistance. 

This strategy involves the use of insecticides with different modes of action in mixtures or in rotations. There are economic problems (e.g., costs of switching chemicals or having storage space for them) associated with this approach, and critical variables in addition to mode of action must be taken into consideration (e.g., mode of resistance inheritance, frequency of mutations, population dynamics of the target species, availability of refuges, and migration). General recommendations are to evaluate resistance patterns at least annually and the need for rotating insecticides at annual or longer intervals.

The status of resistance in the target mosquito populations should be monitored annually in order to provide baseline data for planning programmes and selecting pesticides before control operations begin, detect resistance at an early stage so that management of resistance can be implemented in a timely manner, and continuously monitoring the effects of resistance control strategies on development of resistance. (D147)

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Effects on Non-target Organisms

  • It is essential to remember at all times that the use of insecticides is accompanied by risks to other organisms and to the function of ecosystems. The primary concern is direct toxicity, which may be seen as kills of fish or other species, or as effects which, while not directly lethal, increase the susceptibility of the affected organisms to other causes of morbidity or mortality. Information on the relative risks of pesticides is available (for example on the New York State Department of Health Website). Precautions must be taken when using insecticides to maximise effectiveness while minimising undesirable side effects such as killing of useful non-target species. (D72, D71)
  • Effects on non-target arthropods and other non-target species such as fish are of primary concern in pesticide use. (P32.1.w8)
    • The more selective pesticides, such as oils and microbial larvicides (see: Biological Control for West Nile Virus) should be used over water whenever possible to minimise effects on non-target arthropods. (P32.1.w8)
    • Use of methoprene should be reserved for areas where there are few non-target arthropods. (P32.1.w8)
  • In general larvicides are less toxic to non-target terrestrial organisms than are most available adulticides. (P32.1.w8)
  • Formulation and use of products intended for use around human populations are generally designed to allow a significant margin of safety relative to laboratory animal findings, to allow for unknown species and individual differences which may make some persons particularly susceptible. (P32.1.w8)
  • The local effects of adulticide treatments on non-target organisms such as fish and lobsters should be evaluated. (P32.1.w23)
  • Toxicity of products applied to areas of water may be affected by factors such as the depth, salinity, sediment load, organic content, oxygen content and mixing of the water. (P32.1.w8)
  • Fish deaths due to application of insecticides over water bodies are more likely to occur when the dissolved oxygen content is low (e.g. less than 2.0 ppm), therefore application over such areas should be avoided in such conditions. (P32.1.w23)
  • In addition to direct toxicity effects of chemicals used in mosquito control, the potential for indirect damage to organisms via toxicity to other species should be considered: for example a pesticide may, by killing insects, also reduce food availability for insectivorous species. (P32.1.w8)
    • Use of insecticides may eliminate not only the intended target but also predatory insects which normally limit the numbers of the target organism. (P32.1.w8)
  • All pesticides chosen for mosquito control, if used as directed, pose a very low risk of serious adverse effects for humans exposed by spraying (aerial or ground-level) or other approved forms of application. (P32.1.w8)
    • A small minority of people, and of other animals, may be allergic to a given pesticide; this can greatly increase their response to the pesticide. (P32.1.w8)
    • Risks of chronic exposure to insecticides are small for humans and other homeothermic vertebrates, since most of the insecticides used have thresholds for toxic effects several orders of magnitude higher than the doses to which humans and vertebrates will be exposed in the environment. (P32.1.w8)
Associated techniques linked from Wildpro

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Authors & Referees

Authors Debra Bourne (V.w5)
Referee Suzanne I. Boardman (V.w6); Becki Lawson (V.w26)

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